Abstract

CONTEXT:

The age of puberty has fallen over the past 130 years in industrialized, western countries, and this fall is widely referred to as the secular trend for earlier puberty. The current study was undertaken to test two evolutionary theories: (a) the reproductive system maximizes the number of offspring in response to positive environmental cues in terms of energy balance, and (b) early puberty is a trade-off response for high mortality rate and reduced resource availability.

METHODS:

Using a sample of 22 natural-fertility societies of mostly tropical foragers, horticulturalists, and pastoralists from Africa, South America, Australia, and Southeastern Asia, this study compares indices of adolescence growth and menarche with those of fertility fitness in these non-industrial, traditional societies.

RESULTS:

The average age at menarche correlated with the first reproduction, but did not correlate with the total fertility rate TFR or reproductive fitness. The age at menarche correlated negatively with their average adult body mass, and the average adult body weight positively correlated with reproductive fitness. Survivorship did not correlate with the age at menarche or age indices of the adolescent growth spurt. The population density correlated positively with the age at first reproduction, but not with menarche age, TFR, or reproductive fitness.

CONCLUSIONS:

Based on our analyses, we reject the working hypotheses that reproductive fitness is enhanced in societies with early puberty or that early menarche is an adaptive response to greater mortality risk. Whereas body mass is a measure of resources is tightly associated with fitness, the age of menarche is not.

Age of menarche and the first reproduction. A regression line and 95% confidence limits for the age of the first reproduction as a function of the menarche age in traditional societies. The dashed line connects identical ages on the X and Y axes, demonstrating the constant age gap between the menarche age and first reproduction age.

Puberty and body weight. Upper panels: A regression and 95% confidence limits for the age at menarche as a function of the average adult body weight (left panel) and BMI (right panel). Lower panels: A regression and 95% confidence limits for reproductive fitness as a function of the body weight in both females (left panel) and males (right panel).